1. Field of the Invention
The invention relates to an electric motor-driven servo-drive for use in technical means, especially motor vehicles, with a small electric drive motor having a motor shaft, with a worm of a worm gear pair, the worm preferably being made of plastic and slipped onto the motor shaft, with a driver made of a harder material which transfers higher torques than the plastic of the worm on the motor shaft, with a worm wheel of the worm gear pair preferably being made of plastic and coupled to the worm via teeth which engage one another, the driver on a diameter which is much greater than that of the motor shaft having a worm form-fit element and the worm having a driver form-fit element which engages the worm form-fit element of the driver.
2. Description of Related Art
Electric motor-driven servo-drives of this type are being increasingly used in motor vehicles for a host of applications. One special application is servo-drives on motor vehicle door locks in central locking systems (side doors, rear doors, rear hatch). Many other application are known, from the adjustment of headlights through adjustment of sideview mirrors to headrest and seat height adjustments. Electric motor-driven servo-drives of this type are however also used in many other technical means.
The starting point for the invention is an electric motor-driven servo-drive for use in a central locking system of a motor vehicle (published German Patent Application No. 39 32 268), in which a small electric drive motor is mounted on a base plate and has a motor shaft on which a worm is attached. The worm engages the toothed segment of a worm wheel, the two together form a worm gear pair. In this prior art the worm consists of a metal combination, specifically phosphor bronze, while the worm wheel consists of plastic, especially thermoplastic. In this way certain coefficients of friction between the rows of teeth or annular gears which engage one another are achieved.
In the known prior art the electric drive motor is turned off as soon as the swivel element which is driven via the worm gear pair strikes a rubber stop. This so-called blocking mode however entails comparatively high torques to be transmitted during shut-off which can likewise necessitate making the worm from a metal combination, here from phosphor bronze.
While at the high loads to be expected during operation with respect to the torque to be transmitted, worms generally made of metal are used and are pressed onto the smooth or notched motor shaft, at the lower loads to be expected in operation, for reasons of cost, plastic worms are also willingly used and in the same way they are pressed onto the smooth or notched motor shaft or onto the appropriately flattened motor shaft provided with a corresponding flattened area. This simpler and more economical version of an electric motor-driven servo-drive however for a long time has been prohibited where high torques must be transmitted and mainly with frequent occurrence of the aforementioned blocking mode. If for example a plastic worm with the corresponding opposite flattened area is slipped for example on the flattened motor shaft of a small drive motor (the diameter of the motor shaft is roughly 2 mm) this connection is generally unable to transmit higher torques over a longer time or the torques which occur in the blocking mode.
In a completely different specialty, specifically the knobs for radios and other control devices, the problem of overly high Hertzian stress between flattened metal and plastic parts has been known for decades (U.S. Pat. No. 3,188,124 from 1963). The problem is generally solved there by inserting or pushing a metal reinforcing piece (plate) into the plastic part which is made as a sleeve in the area of the flattening. This prevents occurrence of especially dangerous edge stresses on the edges of the flattened area of the plastic material and routes them into the metal plate. This technique which has been known for decades in control knobs has not had any effect on the area of electric motor-driven servo-drives with worm gear pairs.